Collapsible, extendible double tubular structure

ABSTRACT

A collapsible, extendible element including an elongated thin metallic sheet pre-stressed to curl to form two hollow substantially cylindrical sections when unrestrained from a flattened condition. The elongated thin metallic sheet is adaptable to be rolled into a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition. In several embodiments of the invention two lengthwise rows of slots are provided in the elongated thin metallic sheet that are located near the longitudinal central axis of the sheet and a series of tabs are located on the edges of the sheet that are adapted to be inserted in the slots when the elongated metallic sheet is unrestrained from a flattened condition.

[ Oct. 10, 1972 [541 COLLAPSIBLE, EXTENDIBLE DOUBLE TUBULAR STRUCTURE [72] Inventor: Thomas G. Berry, Silver Spring,

[73] Assignee: Fail-child Hiller Corporation, Montgomery County, Md.

22 Filed: Oct.28, 1970 21 Appl.No.: 84,724

[52] US. Cl ..52/108, 242/54 R, 242/786 [51] Int. Cl. ..E04h 12/16 Field of Search ..52/108,111, 731; 182/41; 29/513 X; 138/l28,162,172,166,151, 156,

[56] References Cited UNITED STATES PATENTS 3,252,173 5/1966 Robinsky ..52/108 3,360,894 l/l968 Sharman et al ..52/108 3,361,377 l/l968 Trexler, Jr. ..242/54 A 774,159 11/1904 Cook ..52/731 3,258,800 7/ 1966 Robinsky ..52/ 108 3,434,674 3/ 1969 Groskopfs ..242/54 A 3,503,164 3/1970 Berry et a1. ..52/108 3,144,104 8/1964 Weir et a1. ..182/41 3,208,478 9/1965 Baines 138/162 554,322 2/1896 Thorpe ..138/115 1,104,714 7/1914 Sonnier ..138/111 FOREIGN PATENTS OR APPLICATIONS 24,338 9/1895 Great Britain ..138/1 15 Primary ExaminerFrank L. Abbott Assistant Examiner-Jxslie A. Braun Attorney-Michael W. York [57] ABSTRACT A collapsible, extendible element including an elongated thin metallic sheet pre-stressed to'curl to form two hollow substantially cylindrical sections when unrestrained from a flattened condition. The elongated thin metallic sheet is adaptable to be rolled into a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition. In several embodiments of the invention two lengthwise rows of slots are provided in the elongated thin metallic sheet that are located near the longitudinal central axis of the sheet and a series of tabs are located on the edges of the sheet that are adapted to be inserted in the slots when the elongated metallic sheet is unrestrained from a flattened condition.

7 Claims, 9 Drawing Figures PATENTEDUCT 10 I972 SHEET 1 0F 3 l NVENTOR. THOMAS a. 5 [/m v BY J4 7708 /v [y PATENTEDUCT 10 m2 SHEET 2 UP 3 INVENTOR, THO/1A8 a 55 95 y COLLAPSIBLE, EXTENDIBLE DOUBLE TUBULAR STRUCTURE BACKGROUND OF THE INVENTION known in the art are disclosed in US. Pat. Nos.

3,360,894 and 3,503,164.

It is highly desirable that an extendible element be readily adapted to be tightly rolled in its flat configuration on a drum or the like so that the coiled or rolled element forms a very compact package and this requires that the element possess a high degree of flexibility as it is about to be rolled on the drum and also during the time it is rolled on the drum. Conversely, when the element has been extended, it is highly desirable that the extended element possess a high degree of rigidity and be resistant to bending or twisting. Prior attempts to obtain this rigidity have included rolling or forming the long narrow sheet into a long thin tube having generally a circular ring shaped cross section. Commonly, tabs and/or slots are provided on the edges of the sheet for assisting in holding the long narrow sheet in its rolled or tubular configuration. Even with such tabs and slots these rolled tubular elements have not provided the degree of resistance to bending and twisting that is desired for many applications. These prior tubular elements have also not provided the moment of inertia that is desired from a narrow low weight sheet. In addition, the edges of the rolled tubular element and the tabs are susceptible to snagging items that may be located adjacent to the tubular elements.

The extendible element of this invention overcomes these problems associated with prior extendible elements and provides an element that has an improved degree of rigidity and a high moment of inertia from a low weight sheet.

SUMMARY OF THE INVENTION This invention relates to extendible elements and more particularly to a collapsible coilable extendible element that is pre-stressed to form cylindrical sections when unrestrained from a flattened condition.

It is accordingly an object of the present invention to provide a collapsible extendible element that is capable of being coiled into a compact package.

It is an object of the present invention to provide a collapsible extendible element that has good rigidity and resists twisting when it is extended.

It is an object of the present invention to provide a collapsible extendible element that has a greater moment of inertia for a given width of material.

It is also an object of the present invention to provide a collapsible extendible element whose edges are not susceptible to snagging adjacent items when the collapsible extendible element is extended.

It is also an object of the present invention to provide a collapsible extendible element Whose edges may be securely connected to the extendible element when the collapsible extendible element is extended and curled.

The present invention provides a collapsible, extendible pre-stressed element comprising an elongated thin sheet pre-stressed to have one edge curl about one of its sides about an axis which is substantially parallel to the longitudinal central axis of the sheet to form a first hollow substantially cylindrical section and to have its opposite edge curl about its opposite side about a third axis that is substantially parallel to the longitudinal central axis of the sheet and lies in a substantially common plane with the first axis and the longitudinal central axis to form a second hollow substantially cylindrical section when the thin sheet is unrestrained from a flattened condition. The elongated thin sheet is adapted to be rolled in a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be hereinafter more fully described with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of the collapsible, extendible element of the present invention as it is being extended;

FIG. 2 is a front elevational view of the structure illustrated in FIG. 1;

FIG. 3 is an end view of the extendible element illustrated in FIGS. 1 and 2;

FIG. 4 is a perspective view of an additional embodiment of the collapsible, extendible element of the invention as it is being extended;

FIG. 5 is a front elevational view of the structure illustrated in FIG. 4;

FIG. 6 is an end view of the extendible element illustrated in FIGS. 4 and 5;

FIG. 7 is a perspective view of the extendible element illustrated in FIGS. 4 through 6 with certain parts broken away illustrating the manner in which tabs located on the edges of the element are inserted into slots in the element when the element is extended;

FIG. 8 is a perspective view of an additional embodiment of the collapsible, extendible element of the invention as it is being extended; and

FIG. 9 is an end view of the extendible element illustrated in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 through 2 the collapsible, extendible device of the invention designated generally as 10 comprises an elongated thin metallic sheet 11 that is pre-stressed so that one of its edges which will be referred to as its left edge 12 will curl upward about one side of the sheet about an axis A that is parallel to the longitudinal central axis B of the sheet to form a hollow substantially cylindrical section 13 when the sheet is unrestrained and so that its other edge which will be referred to as its right edge 14 will curl under about the opposite side of the sheet about an axis C that is also parallel to the longitudinal central axis B to form a hollow substantially cylindrical section 15 that is located adjacent to the section 13 when the sheet is not restrained in its flattened condition. In the preferred embodiment the axis A and the axis C which are the longitudinal axes of the respective substantially cylindrical sections 13 and 15 lie in substantially the same plane indicated by the line D as the longitudinal axis 8 of the sheet and this permits the extended sheet 11 to resist bending in a direction parallel to the plane D. The elongated thin metallic sheet 11 is formed from a resilient material such as spring steel or beryllium copper having sufficient flexibility to enable the sheet to be opened flat without a permanent set so that the energy produced by the pre-stressing is stored by the sheet and is available when the sheet is unrestrained so that its edges will curl in the manner previously described. The elongated metallic sheet 11 is adapted to be rolled into a coil about an axis generally transverse to its longitudinal axis in its flattened or restrained condition about a cylindrical drum 16 that is rotatably mounted on a shaft 17 that passes through the long axis of the drum.

A tube guide designated generally as 18 is located in front of the drum l6 and comprises rollers 19 and 20 rotatably mounted on respective shafts 21 and 22. The rollers 19 and 20 have respective dished surfaces 23 and 24 that substantially match the exterior shape of the respective cylindrical sections 13 and 15. The rollers 19 and 20 are located opposite each other and they are spaced apart by a sufficient distance so that the distance measured between the respective surfaces 23 and 24 of the rollers is approximately equal to the width of the extended sheet 11 when it has curled to form the two cylindrical sections 13 and 15, so that the guide rollers assist the sheet in assuming its proper configuration as it is being extended. The tube guide 18 also comprises a guide wheel 25 that is rotatably mounted on a shaft 26 above the extended portion of the sheet 11 and is adapted to contact the underside of the left edge portion of the sheet as it is being extended so that it guides and assists the sheet in assuming its proper extended configuration. The tube guide 18 also comprises a guide wheel 27 that is substantially identical to the guide wheel 25 that is rotatably mounted on a shaft 28 below the extended portion of the sheet 1 l and is adapted to contact the upperside of the right edge portion of the sheet as it is being extended to assist the sheet in assuming its proper extended configuration. It will be appreciated that the shafts 17, 21, 22, 26, and 28 can be mounted on a suitable frame (not shown) so that the respective drum 16, rollers 19 and 20, and guide wheels 25 and 27 are held in their desired relationships.

As best illustrated in FIG. 3 when the sheet 11 is extended the hollow substantially cylindrical section 13 and the hollow substantially cylindrical section 15 have substantially circular cross sections so that the cross section of the entire extended curled sheet 11 is substantially shaped like a figure 8. When the sheet 11 is extended, the left portion of the sheet has curled upward and inward so that the left edge of the sheet 12 is in contact with the upper surface of the central portion of the sheet. In a similar manner when the sheet 11 is extended, the right portion of the sheet has curled downward and inward so that the right edge of the sheet 14 is in contact with the lower surface of the central portion of the sheet.

Referring to FIGS. 4 and a second form of extendible device is illustrated and generally designated as 29 that comprises an elongated pre-stressed thin metallic sheet 30 that is partially wound in a coil about an axis generally transverse to its longitudinal axis in a flattened condition upon a drum 31 that is rotatably mounted on a shaft 32 in a manner similar to that previously described with reference to the embodiment of the invention illustrated in FIGS. 1 and 2. The elongated thin metallic sheet 30 is formed from the same type of resilient material such as spring steel or beryllium copper as the sheet 11 illustrated in FIGS. 1 through 3 and thus the sheet has a sufficient degree of flexibility to enable the sheet to be opened flat without a permanent set so that the energy produced by the pre-stressing is stored by the sheet and is available to cause the sheet to curl in the manner hereinafter described when the sheet is unrestrained. The sheet 30 has a series of generally pointed tabs 33 located at intervals along and extending from what will be referred to as its left edge 34 and a series of substantially identical generally pointed tabs 35 located at intervals along and extending from what will be referred to as its right edge 36. The sheet 30 has a row of spaced slots 37 that are located substantially parallel to the longitudinal central axis of the sheet and slightly toward the right edge 36 from the longitudinal central axis. The slots 37 are adapted to receive the tabs 33 located on the left edge 34 of the sheet. The slots 37 are spaced in mating alignment with the opposite corresponding tabs 34 so that they can readily receive these tabs when the sheet 30 is unrestrained and curls. The sheet 30 also has a row of spaced slots 38 that are substantially identical to the slots 37 and are located substantially parallel to the longitudinal central axis of the sheet and slightly toward the left edge 34 of the sheet from its longitudinal central axis. The slots 37 are spaced in mating alignment with the opposite corresponding tabs 35 and they are adapted to readily receive these tabs as the sheet curls when it is unrestrained. The sheet 30 is prestressed in a manner similar to that of the embodiment illustrated in FIGS. 1 through 3, so that its left edge 34 will curl upward about an axis E that is substantially parallel to the longitudinal central axis F of the sheet to form a hollow substantially cylindrical section 39 when the sheet is unrestrained and so that its right edge 36 will curl downward about an axis G that is also substantially parallel to the longitudinal axis F to form a substantially cylindrical section 40 that is located adjacent to the cylindrical section 39 when the sheet 30 is not restrained in its flattened rolled condition. In the preferred embodiment the axes E and G which are the longitudinal axes of the respective substantially cylindrical sections 39 and 40 lie in substantially the same plane indicated by the line H as the longitudinal axis F of the sheet 30.

As the left edge 34 of the sheet 30 curls when the sheet is unrolled, the tabs 33 are forced into the slots 37 and as the right edge 36 curls when the sheet is unrestrained the tabs 35 are forced into the slots 38 so that the edges of the extended sheet are rigidly secured to the central portion of the sheet.

A tube guide designated generally as 41 that is similar to the previously described tube guide 18 is located in front of the drum 31 and comprises rollers 42 and 43 rotatably mounted on respective shafts 44 and 45. The rollers 42 and 43 have respective dished surfaces 46 and 47 that substantially match the exterior shape of the respective cylindrical sections 39 and 40. The rollers 42 and 43 are located opposite each other and they are spaced apart by a sufficient distance so that the distance measured between the respective surfaces 46 and 47 of the rollers is approximately equal to the width of the extended sheet when it has curled to form the two cylindrical sections 39 and 40, so that the guide rollers assist the sheet in assuming its proper configuration as it is being extended. The tube guide 41 also comprises a guide wheel 48 that is rotatably mounted on a shaft 49 above the extended portion of the sheet 30 and is adapted to contact the underside of the left edge portion of the sheet as it is being extended so that it assists the sheet in assuming its proper extended configuration. The tube guide 41 also comprises a guide wheel 50 that is substantially identical to the guide wheel 48 that is rotatably mounted on a shaft 51 below the extended portion of the sheet 30 and is adapted to contact the upperside of the right edge portion of the sheet as it is being extended to assist the sheet in assuming its proper extended configuration. The shafts 32, 44, 45, 49, and 51 can be mounted on a suitable frame (not shown) so that the respective drum 31, rollers 42 and 43, and guide wheels 48 and 50 are held in their desired relationships.

As best illustrated in FIGS. 6 and 7 when the sheet 30 is extended the hollow substantially cylindrical section 39 and the hollow substantially cylindrical section 40 have substantially circular cross sections so that the cross section of the entire extended sheet is substantially shaped like a figure 8. When the sheet 30 is extended, the tabs 33 that are on the left edge of the sheet have been inserted through the slots 37 in the upper side of the sheet and they extend into the interior of the substantially cylindrical section 40. In a similar manner when the sheet 30 is extended, the tabs that are on the right edge of the sheet have been inserted into the slots 38 in the underside of the sheet and they project into the interior of the cylindrical section 39. As a result, the cylindrical sections 39 and 40 share a common wall portion located between the slots 37 and 38 designated as 51 and this gives the extended sheet 30 much greater rigidity than if the tabs 33 were inserted into the slots 38 and the tabs 35 were inserted into the slots 37 so that there was no such commonly shared wall portion.

An additional embodiment of the iny ention designated generally as 52 that is similar to the e ni'bodiinent illustrated in FIGS. 4 through 7 is illustrated in FIGS. 8 and 9 and comprises an elongated pre-stressed thin metallic sheet 53 partially rolled into a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition upon a drum 54 that is rotatably mounted on a shaft 55. The thin metallic sheet 53 is formed from a resilient material such as spring steel or beryllium copper and is pre-st ressed in a manner similar to the previously described embodiments so that one of its edges which will be referred to as its left edge 56 will curl upward about an axis I that is substantially parallel to the longitudinal axis J of the extended sheet to form a hollow substantially cylindrical section 57 when the sheet is unrestrained. The metallic sheet 53 is also pre-stressed so that its other edge or right edge 58 will curl downward about an axis K that is also substantially parallel to the longitudinal axis .I to form a hollow substantially cylindrical section 59 that is located adjacent to the cylindrical section 57 when the sheet is unrestrained.

Pointed tabs 60 are located at intervals along the left edge 56 of the sheet 53 and these tabs fit into corresponding slots 61 that are in mating alignment with and are adapted to receive the tabs when the sheet is unrestrained and curls. The slots 61 are located in a row that lies substantially parallel to the longitudinal central axis of the sheet and slightly toward the right edge of the sheet from the sheets longitudinal central axis. A series of pointed tabs 62 are also located at intervals along the right edge 58 and these tabs fit into corresponding slots 63 that are in mating alignment with and that are adapted to receive the tabs as the sheet curls. The slots 63 are located in a row that lies substantially parallel to the longitudinal central axis of the sheet and slightly toward the left edge of the sheet from the longitudinal central axis of the sheet. It will be appreciated that for some applications the tabs 60 and 62 and the slots 61 and 63 of the embodiment illustrated in FIGS. 8 and 9 will be unnecessary and thus they can be omitted.

A tube guide generally designated 64 that is similar to the tube guides of the previously described embodiments is located in front of the drum 54. The tube guide 64 comprises rollers 65 and 66 that are rotatably mounted on the respective shafts 67 and 68 and are spaced apart so that the distance between the surfaces 69 and 70 of the respective rollers is approximately equal to the width of the extended curled portion of the sheet 53. The tube guide 64 also includes a guide wheel 71 that is rotatably mounted on a shaft 72 and is located above and is in contact with the left curled section 57 of the sheet. The tube guide also includes another guide wheel 73 that is rotatably mounted on a shaft 74 and is located below and is in contact with the right curled section 59 of the sheet. It will be appreciated that the shafts 55, 67, 68, 72 and 74 can be mounted on a suitable frame (not shown) so that the drum 54, the rollers 65 and 66, and the wheels 71 and 73 can be located in their desired relationships. The tube guide 64 is adapted to assist the sheet 53 in curling properly to assume its proper configuration as the sheet is unwound from the drum 54.

In FIG. 9 it can be seen that when the sheet 53 is unrestrained! and extended the extended hollow substantially cylindrical sections 57 and 59-have substantially elliptic cross sections so that the cross section of the entire extended curled sheet 53 is substantially shaped like a flattened figure 8. The tabs 60 on the left edge of the sheet 53 have been inserted into the slots 61 from above the sheet and extend into the interior of the right curled cylindrical section 59 when the sheet is unrestrained and has curled. In a similar manner the tabs 62 on the right edge of the sheet 53have been inserted into the slots 63 from below the sheet and extend into the interior of the left curled cylindrical section 57 when the sheet is unrestrained and has curled. The cylindrical sections 57 and 59 also share a common wall portion 75 located between the slots 61 and 63 and this gives the extended sheet improved rigidity.

In order to utilize the invention, the sheet 11, the sheet 30, or the sheet 53 is generally kept coiled in a compact package on the respective drum 16, 31 or 54 until it is desired to extend the appropriate sheet. To extend the sheet 11,30 or 53 the appropriate drum 16, 31 or 54 is rotated by suitable means known in the art (not shown) so that the sheet extends outward from the drum and begins to curl due to the pre-stressing and the action of the appropriate tube guide 18, 41 or 64. As the sheet 11 is extended outward, the pre-stressing causes the sheet to curl and the edges 12 and 14 to contact the central portion of the sheet so that two cylindrical sections 13 and 15 are formed that are located adjacent to each other and the extended sheet has a figure 8 cross section. As the sheet is extended outward, the pre-stressing causes the sheet to curl and the tabs 33 to enter the slots 37 and causes the tabs to enter the slots 38 so that the extended curled sheet has a rigid figure 8 cross section. In a like manner as the sheet 53 is extended, the pre-stressing causes the tabs 60 to enter the slots 61 and the tabs 62 to enter the slots 63 so that the extended curled sheet has a rigid flattened figure 8 cross section. Since the sheets 11, 30 and 53 curl so that their edges contact the central portion of the sheet the extended sheet presents no edge that might snag items that might be adjacent to the sheet.

It will be appreciated that an extendible sheet of relatively long length can be stored in a compact package since the sheet 11, 30 or 53 is stored in its flattened condition in a relatively compact coil and in view of this such sheets can be extended to form rigid booms and the like of comparatively great lengths. If it is desired to collapse the extended sheet 11, 30 or 53 the drum 16, 31 or 54 is rotated in the appropriate manner by means familiar to those skilled in the art (not shown) so that the appropriate sheet is rewound on the appropriate drum. As rewinding takes place the sheet 1 1, 31 or 54 is flattened out as it is being rewound and in the case of the sheets 31 and 54 this results in the tabs 33 and 35 being pulled from the respective slots 37 and 38 and the tabs 60 and 62 being pulled from the respective slots 61 and 63.

The figure 8 cross section of the embodiment of the invention illustrated in FIGS. 1 through 7 and the flattened figure 8 cross section of the embodiment illustrated in FIGS. 8 and 9 gives the extended sheet 11, the extended sheet 30, or the extended sheet 35 a high moment of inertia about their Y axes. In view of these improved cross sections the sheets 11, 30 and 53 when they are extended provide resulting structures that possess greater rigidity from a given weight of material than those previously known in the art. The location of the respective cylindrical sections 13 and 15, 39 and 40, 57 and 59 adjacent to each other and the sharing of the common wall portion 51 and 75 gives improved rigidity and results in structures that tend to resist twistmg.

Although the various embodiments of the invention have been shown as having tabs on both edges of the sheet or no tabs at all, it should be appreciated that for some applications, it might be desirable to have tabs on only one edge of the sheet. It will also be appreciated that although in the preferred embodiments the extendible sheet is formed from steel or copper, the extendible sheet may also be formed by suitable known techniques from a suitable plastic such as that known by the trademark Mylar.

Although the invention has been described with reference to certain preferred embodiments, it will be understood that variations and modifications may be made within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A collapsible, extendible pre-stressed device comprising an elongated thin pre-stressed sheet defining a curled extended structure having a first hollow substantially cylindrical section formed by one edge of said elongated thin sheet being curled about one side thereof about a first axis which is substantially parallel to the longitudinal central axis of said elongated thin sheet and having a second hollow substantially cylindrical section located adjacent to said first substantially cylindrical section formed by the opposite edge of said elongated thin sheet being curled about the opposite side thereof about a third axis which is substantially parallel to the longitudinal central axis of the sheet and which lies in a substantially common plane with the first axis and the longitudinal central axis of said sheet when said elongated thin sheet is unrestrained from a flattened condition, said elongated thin sheet being collapsible to be rolled in a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition, first guide means in contact with one of said hollow substantially cylindrical sections and second guide means in contact with the other hollow substantially cylindrical section for assisting said elongated thin sheet in assuming its proper extended configuration, said first and said second guide means having respective inner surfaces which are separated by a distance approximately equal to the width of said elongated thin sheet when said elongated thin sheet has curled to form said first and second hollow substantially cylindrical sections.

2. The collapsible, extendible pre-stressed device of claim 1 wherein said first and second substantially cylindrical sections each have a substantially circular cross section and the cross section of said entire elongated thin sheet is substantially shaped like a figure 8 when said thin sheet is unrestrained from a flattened condition.

3. The collapsible, extendible pre-stressed device of claim 1 wherein said first and second substantially cylindrical sections each have a substantially eliptic cross section and the cross section of said entire elongated thin sheet is substantially shaped like a flattened figure 8 when said thin sheet is unrestrained from a flattened condition.

4. The collapsible, extendible pre-stressed device of claim 1 wherein one edge of said elongated thin sheet is in contact with the surface of one side of the central portion of said elongated thin sheet and the other edge is in contact with the surface on the opposite side of the central portion of said elongated thin sheet when said thin sheet is unrestrained from a flattened condition.

5. The collapsible, extendible pre-stressed device of claim 4 wherein said elongated thin pre-stressed sheet has spaced tabs along the longitudinal edges thereof and two rows of slots located substantially parallel to and near the central longitudinal axis of said sheet, said slots being adapted to receive said tabs as said sheet curls when unrestrained from a flattened condition.

6. The collapsible, extendible pre-stressed device of claim 5 wherein said first and second cylindrical sections share a common wall portion and said common wall portion is located between said two rows of slots. 

1. A collapsible, extendible pre-stressed device comprising an elongated thin pre-stressed sheet defining a curled extended structure having a first hollow substantially cylindrical section formed by one edge of said elongated thin sheet being curled about one side thereof about a first axis which is substantially parallel to the longitudinal central axis of said elongated thin sheet and having a second hollow substantially cylindrical section located adjacent to said first substantially cylindrical section formed by the opposite edge of said elongated thin sheet being curled about the opposite side thereof about a third axis which is substantially parallel to the longitudinal central axis of the sheet and which lies in a substantially common plane with the first axis and the longitudinal central axis of said sheet when said elongated thin sheet is unrestrained from a flattened condition, said elongated thin sheet being collapsible to be rolled in a coil about an axis generally transverse to its longitudinal axis in a generally flattened condition, first guide means in contact with one of said hollow substantially cylindrical sections and second guide means in contact with the other hollow substantially cylindrical section for assisting said elongated thin sheet in assuming its proper extended configuration, said first and said second guide means having respective inner surfaces which are separated by a distance approximately equal to the width of said elongated thin sheet when said elongated thin sheet has curled to form said first and second hollow substantially cylindrical sections.
 2. The collapsible, extendible pre-stressed device of claim 1 wherein said first and second substantially cylindrical sections each have a substantially circular cross section and the cross section of said entire elongated thin sheet is substantially shaped like a figure ''''8'''' when said thin sheet is unrestrained from a flattened condition.
 3. The collapsible, extendible pre-stressed device of claim 1 wherein said first and second substantially cylindrical sections each have a substantially eliptic cross section and the cross section of said entire elongated thin sheet is substantially shaped like a flattened figure ''''8'''' when said thin sheet is unrestrained from a flattened condition.
 4. The collapsible, extendible pre-stressed device of claim 1 wherein one edge of said elongated thin sheet is in contact with the surface of one side of the central portion of said elongated thin sheet and the other edge is in contact with the surface on the opposite side of the central portion of said elongated thin sheet when said thin sheet is unrestrained from a flattened condition.
 5. The collapsible, extendible pre-stressed device of claim 4 wherein said elongated thin pre-stressed sheet has spaced tabs along the longitudinal edges thereof and two rows of slots located substantially parallel to and near the central longitudinal axis of said sheet, said slots being adapted to receive said tabs as said sheet curls when unrestrained from a flattened condition.
 6. The collapsible, extendible pre-stressed device of claim 5 wherein said first and second cylindrical sections share a common wall portion and said common wall portion is located between said two rows of slots.
 7. The collapsible, extendible pre-stressed device of claim 1 further comprising third guide means in contact with one edge portion of said elongated thin sheet and fourth guide means in contact with the opposite edge portion of said elongated thin sheet for assisting said elongated thin sheet in assuming its proper extended configuration. 